French Patent Application FR-A-2,723,837 discloses automatically breaking implants made up of nuts which are divided into two sections separated is by a peripheral central zone which breaks at a predetermined tightening torque. For the small sizes of such automatically breaking implants, the end of the screwdriver engaged in the corresponding impression of the implant is of very small dimensions, its diameter being a few millimeters. On account of this small size of the end of the screwdriver, it can happen, as a result of insufficient mechanical strength when the breaking torque is reached, or even before it is reached, that this end breaks. It is then necessary to replace the screwdriver.
Moreover, when this automatically breaking nut has reached its breaking torque without the end of the screwdriver being broken, surgeons often tend to want to give an additional turn to the remaining part of the nut, screwed into the implant, and this after the nut has broken. This additional tightening is undesirable and can cause the end of the screwdriver to break because the torque exerted is greater than the breaking torque.
One solution to this problem is to provide a tightening instrument which is designed such that it can no longer be used once the defined breaking torque has been reached.
Document U.S. Pat. No. 4,838,264 proposes instruments of this type intended for surgical applications. In a first illustrative embodiment, a cap provided with a grip is engaged on the head of a screw. It includes a zone of less resistance which breaks under torsion once the torque applied by the cap exceeds a given value. The cap is therefore used only once. In a second illustrative embodiment, the tightening instrument is made up of two parts which are connected via a solid element of polygonal cross section provided with a zone of less resistance which breaks under torsion when the torque applied exceeds a given value. This solid component can therefore be used only once. For surgical applications, this instrument has the disadvantage that it cannot be immediately reused since the component which can break off has to be replaced. Moreover, the two instruments which have just been described comprise disposable rupturing components which are relatively voluminous and are expensive to produce.
In general mechanics, instruments area known in which the torque-limiting function is ensured by a pin which connects two elements of the instrument and which breaks by shearing once a predetermined applied torque value is reached. In this regard, mention may be made of documents U.S. Pat. Nos. 4,703,677 and 3,753,625. In these, the rupturing component is of small dimensions and thus not expensive. However, the known configurations using a pin are not applicable without difficulty to surgery. In particular, there is nothing to retain the pieces of the broken pin, and these pieces risk falling into the operating site when the instrument is used.